From 139eba3d97dce05216251d7d34365cc2b1d37e4c Mon Sep 17 00:00:00 2001
From: STerliakov <terlya.stas@gmail.com>
Date: Sun, 2 Nov 2025 15:02:48 +0100
Subject: [PATCH] Do not write "final" cache results for fast containers inside
 lambdas

---
 mypy/checkexpr.py                           | 10 ++++++++--
 test-data/unit/check-inference-context.test | 14 ++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index 3eb54579a050..d87b8dc8c95a 100644
--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -5082,7 +5082,10 @@ def fast_container_type(
             self.resolved_type[e] = NoneType()
             return None
         ct = self.chk.named_generic_type(container_fullname, [vt])
-        self.resolved_type[e] = ct
+        if not self.in_lambda_expr:
+            # We cannot cache results in lambdas - their bodies can be accepted in
+            # error-suppressing watchers too early
+            self.resolved_type[e] = ct
         return ct
 
     def _first_or_join_fast_item(self, items: list[Type]) -> Type | None:
@@ -5317,7 +5320,10 @@ def fast_dict_type(self, e: DictExpr) -> Type | None:
             self.resolved_type[e] = NoneType()
             return None
         dt = self.chk.named_generic_type("builtins.dict", [kt, vt])
-        self.resolved_type[e] = dt
+        if not self.in_lambda_expr:
+            # We cannot cache results in lambdas - their bodies can be accepted in
+            # error-suppressing watchers too early
+            self.resolved_type[e] = dt
         return dt
 
     def check_typeddict_literal_in_context(
diff --git a/test-data/unit/check-inference-context.test b/test-data/unit/check-inference-context.test
index a41ee5f59670..b5b5d778d90f 100644
--- a/test-data/unit/check-inference-context.test
+++ b/test-data/unit/check-inference-context.test
@@ -709,6 +709,20 @@ f(
     A(), r=B())
 [builtins fixtures/isinstance.pyi]
 
+[case testLambdaWithFastContainerType]
+from collections.abc import Callable
+from typing import Never, TypeVar
+
+T = TypeVar("T")
+
+def f(a: Callable[[], T]) -> None: ...
+
+def foo(x: str) -> Never: ...
+
+f(lambda: [foo(0)])  # E: Argument 1 to "foo" has incompatible type "int"; expected "str"
+f(lambda: {"x": foo(0)})  # E: Argument 1 to "foo" has incompatible type "int"; expected "str"
+[builtins fixtures/tuple.pyi]
+
 
 -- Overloads + generic functions
 -- -----------------------------